The present invention is directed to power factor correcting electrical converter apparatuses. Power factor correcting electrical converter apparatuses are known, as discussed in U.S. Pat. No. 5,777,866 to Jacobs, et al, for "Power Factor Control for Switched Mode Rectifiers with Improved Representing of Currents in EMI Capacitive Elements"; which is assigned to the assignee of the present application (hereinafter referred to as "the '866 Patent").
A power factor correcting electrical converter apparatus receives an alternating electrical input from a power source. The apparatus includes an input electromagnetic interference (EMI) filter connected with the power source. A rectifying circuit is connected with the EMI filter for receiving the input and generating a rectified voltage. To reduce the cost of the EMI filter, a capacitor is usually placed across the rectified voltage at the rectifying circuit. However, such a capacitor introduces more input current distortion because it prevents current flowing into the rectifier circuit (usually implemented as a diode bridge) whenever the voltage across the capacitor is greater than the value of the input voltage. A switching mode converter, usually a boost type, is connected between the rectifying circuit and an output to provide a regulated power to the output.
In order to draw a low-distortion input current, the '866 Patent teaches constructing such a power factor control converter using a capacitor sense arrangement to provide a feed-forward signal to a control loop. In particular, the '866 Patent teaches using a sense capacitor to provide a current to a current amplifier driving a pulse width modulator to control a power switch connecting a load to the converter apparatus.
The feed-forward arrangement taught by the '866 Patent is intended to reduce current distortion around zero-crossing of the input current signal. Current distortion is reduced by the enhanced converter of the '866 Patent. However, in that prior art converter apparatus extra protection circuitry is required because the apparatus is not controllable at very light loads. Further improvement of the apparatus' performance is desirable.